1. Field of the Invention
The present invention relates optical pattern recognition systems and, in particular, to optical pattern recognition systems using photorefractive materials.
2. Description of the Prior Art
Optical pattern recognition is important for applications in many fields including machine vision, target tracking, missile homing and guidance, automation in space applications and unmanned space exploration. One of the most significant techniques used in optical pattern recognition is the acquisition of two dimensional --or 2D--correlation signals between two images, such as a stored object image and an input reference image, with an optical correlator. The intensity of the correlation signal is often used to indicate the similarity or correlation between the object and reference images.
Conventional optical pattern recognition systems use optical correlators which are only able to produce correlation signals whose intensity is limited by the diffraction efficiency of the recording medium. In particular, the correlation signal is often low due to poor diffraction efficiency of the recording medium.
Although the resultant correlation signal may then be amplified by conventional means, such post correlation amplification amplifies the correlation noise as well as the correlation signal.
One conventional optical correlator is the Vander Lugt holographic matched filter correlator described by A. B. Vander Lugt in the article "Signal Detection by Complex Spatial Filtering", IEEE Trans. Inf. Theory, IT-10, pp 139-145 (1964). The Vander Lugt optical correlator process uses a preliminary step in which an interference pattern based on the object image is recorded in a holographic plate by interference of an object beam and a plane wave reference beam. An addressing or correlation step is then used in which a reference image is applied to the previously recorded holographic interference pattern to generate a correlation signal.
In the recording process, the object image is Fourier transformed with a first Fourier transform--or FTL--lens and interfaced with a plane wave reference beam at a specific angle of incidence. The holographic plate is then developed to form the hologram. In the addressing process, the plane wave reference beam is removed and a reference image is applied to and diffracted by the interference pattern of the hologram. The diffracted beam is applied to a second FTL lens at the same angle of incidence and detected. The detected signal is proportional to the cross-correlation between the object image recorded in the hologram and the reference image later applied to the hologram. If the reference image is the same as the recorded input object, an auto-correlation signal is obtained. The diffraction efficiency of the holographic matched filter correlator is dependent on the holographic recording material used.
What is needed is an optical pattern recognition technique in which the intensity of the recognition signal is greater than that obtainable with conventional techniques.